Can Sugar Substitutes Make You Fat?

When it comes to dieting, most of us are willing to resort to a trick or two to help us curb our appetite and eat less — drinking water to fill up when we're hungry, for example, or opting for artificial sweeteners instead of sugar to get the same satisfying sweetness without the offending calories. But new research suggests that the body is not so easily fooled, and that sugar substitutes are no key to weight loss — perhaps helping to explain why, despite a plethora of low-calorie food and drink, Americans are heavier than ever.

In a series of experiments, scientists at Purdue University compared weight gain and eating habits in rats whose diets were supplemented with sweetened food containing either zero-calorie saccharin or sugar. The report, published in Behavioral Neuroscience, presents some counterintuitive findings: Animals fed with artificially sweetened yogurt over a two-week period consumed more calories and gained more weight — mostly in the form of fat — than animals eating yogurt flavored with glucose, a natural, high-calorie sweetener. It's a continuation of work the Purdue group began in 2004, when they reported that animals consuming saccharin-sweetened liquids and snacks tended to eat more than animals fed high-calorie, sweetened foods. The new study, say the scientists, offers stronger evidence that how we eat may depend on automatic, conditioned responses to food that are beyond our control.

What they mean is that like Pavlov's dog, trained to salivate at the sound of a bell, animals are similarly trained to anticipate lots of calories when they taste something sweet — in nature, sweet foods are usually loaded with calories. When an animal eats a saccharin-flavored food with no calories, however — disrupting the sweetness and calorie link — the animal tends to eat more and gain more weight, the new study shows. The study was even able to document at the physiological level that animals given artificial sweeteners responded differently to their food than those eating high-calorie sweetened foods. The sugar-fed rats, for example, showed the expected uptick in core body temperature at mealtime, corresponding to their anticipation of a bolus of calories that they would need to start burning off — a sort of metabolic revving of the energy engines. The saccharin-fed animals, on the other hand, showed no such rise in temperature. "The animals that had the artificial sweetener appear to have a different anticipatory response," says Susan Swithers, a professor of psychological sciences at Purdue University and a co-author of the study. "They don't anticipate as many calories arriving." The net result is a more sluggish metabolism that stores, rather than burns, incoming excess calories.

Swithers stops short of saying that the animals in her study were compelled to overeat to compensate for phantom calories. But she says that the study does suggest artificial sweeteners somehow disrupt the body's ability to regulate incoming calories. "It's still a bit of a mystery why they are overeating, but we definitely have evidence that the animals getting artificially sweetened yogurt end up eating more calories than the ones getting calorically sweetened yogurt."

Though it's premature to generalize based on animal results that the same phenomena would hold true in people, Swithers says, she notes that other human studies have already shown a similar effect. A University of Texas Health Science Center survey in 2005 found that people who drink diet soft drinks may actually gain weight; in that study, for every can of diet soda people consumed each day, there was a 41% increased risk of being overweight. So even though her findings were in animals, says Swithers, they could lead to a better understanding of how the human body responds to food, and explain why eating low-calorie foods doesn't always lead to weight loss. "There is lots of evidence that we learn about the consequences about eating food," she says. "And we have physiological responses to food that are conditioned."

So does that mean you should ditch the artificial sweeteners and welcome sugar back into your life? Not exactly. Excess sugar in the diet can lead to diabetes and heart disease, even independent of its effect on weight. But it's worth remembering that when it comes to counting calories, it's not just the ones you eat that you have to worry about. The calories you give up matter too, and they may very well reappear in that extra helping of pasta or dessert that your body demands. Your body may actually be keeping better count than you are.

FAT Calories vs. CARB Calories - Which is Worse?

If you're one of millions of Americans trying to lose weight, you probably wish you had a more effective diet than the one you're on now. And if you're wondering whether Dean Ornish's low-fat diet will help you shed pounds better than Dr. Atkins' low-carb menu, the answer is simple: it doesn't matter.

Scientists know that on a molecular level, different types of starch and different types of fat have varying effects when they hit the body. But in terms of weight loss, low-fat diets and low-carb diets overall are equally effective (and, most of the time, neither will help you keep the weight off long-term), says Walter Willett, chair of the department of nutrition at Harvard School of Public Health. Here's why.

Q:Will eating a calorie of fat make you fatter than eating a calorie of carbohydrate?

A: From many kinds of studies conducted over years, we are quite confident now that a calorie from fat will cause a similar amount of weight gain as a calorie from carbohydrate. There are some interesting questions about whether eating carbohydrate calories versus fat calories will make you eat more calories, but based on what you put into your mouth, it's pretty clear that the source of the calories is really not important.

[Whether fats or carbohydrates are more filling] is one issue that's been raised — but it's been raised on both sides. The best way to get to the bottom line is to look at long-term studies where we randomize people to a high-fat/low-carb diet or to a low-fat/high-carb diet and follow them for at least a year or more. That kind of study takes into account the possibility that one kind of diet provides more satiety; so, over the long run you would see more weight loss on that diet. But those studies — half a dozen or more have been done — show quite clearly that the percentage of calories from fat has very little effect on long-term weight loss.


One possible footnote to this issue relates to some recent evidence on trans fats. We have seen in our studies that people who eat more trans fats seem to gain more weight, even when the total calories are the same. I was a little skeptical about that, in part because we're not quite sure we can measure calorie [intake] precisely enough. [It's hard for people to track their portion sizes to the gram, or even be sure of exactly what they're eating, especially if they ever eat out.] But in recent five-year feeding study in monkeys — they're animals so you can control their diets — the monkeys on the high-trans-fat diet gained more weight. They gained about 7% of their body weight over a five-year period, compared to the monkeys on a low-trans-fat diet, who gained about 1.5% of their body weight over five years.

So there may be something more complicated going on there. But there's not any good data [to explain why a calorie of trans fat should cause more weight gain than a calorie of something else]. It may be that on the high-trans-fat diet you're more likely to push those calories into your fat cells rather than your muscle cells — and muscles burn calories 24 hours a day. In the long run, that could make a difference in weight gain. But that's speculation. We're really not sure.

We've now looked at over 250,000 men and women for up to 30 years, and we [also] haven't seen that the percentage of calories from fat or from carbohydrates in your diet makes any difference in relation to heart attacks, various cancers or stroke. Having said that, the type of fat is very important, and so is the type of carbohydrate. So we find that trans fats, again, are particularly harmful with regard to type 2 diabetes and heart disease. On the other hand, unsaturated fats are actually beneficial in terms of reducing the risk of heart disease and type 2 diabetes. It's the same with carbohydrates. The total amount is not important. But high intake of refined starch and sugar is related to a higher risk of heart disease and diabetes, whereas high-fiber whole-grain carbohydrates are related to a lower risk. That's not too surprising, as we know that high intakes of sugar and refined starch have an adverse effect on blood glucose levels.

So the quality of the diet is really important, but just looking at fat versus carbohydrate misses where all the action is.